Typical elevators known to the art control elevator door motion with analog systems which vary the elevator door motor voltage as a function of the position of the door, which in turn is controlled either by cam switches, switches positioned along the path of the door travel, or potentiometers responsive to door position. In such systems, the inherent time constants in the analog circuits, the necessary time for relays to operate, and the mechanical inertia of the door system as a whole provide inherent lags that necessarily smooth out the response to discrete and time variant signals. And, step function voltage changes are similarly responded to in a lag fashion, which avoids excessive jerky response, overshooting situations, and the like. Such systems therefore have very wide tolerances, and a particular design will normally work on any elevator door of a given design and size. But such prior systems are subject to all the frailties of mechanical systems, such as wear and misalignment: frequent adjustments may be required in order to derive proper operation of the door, diagnosis of faulty door operation is difficult, and maintenance is costly. And, such systems are not well suited to providing various optimum characteristics to suit a variety of constraints imposed upon the door under all circumstances.
In more sophisticated door controls, door behavior may be precisely dictated in accordance with desired response, examples of which are described in detail with respect to related inventions, hereinafter. However, the most sophisticated of such systems may operate in response to discrete states and discrete values, such as digital representations of speed, motor current, and the like. Since digital systems can be very precise, so that controlling door response by means of a dictated velocity in accordance with a desired door motion profile can be very carefully controlled in a closed loop fashion. In order to derive maximum benefit from such a system, it is desirable to dictate velocity in accordance with a door motion profile which causes the door to move (open or close) at the highest possible speed consistent with smooth operation and safety constraints, such as inertial limitations depending upon the speed and mass of the door. However, as is known, any error-response system requires a lag between the actual condition and the commanded condition in order to create the command to cause the motion desired. For a minimal lag, maximum gain is required. However, if there are perturberances in the system, such as irregularities in the elevator door mechanism chain drive, variations in friction or other impediments to door motion, and the like, a higher gain will cause a higher system response which can result in oscillations about such perturberances. This defeats the desired result of smoothness. On the other hand, if smoothness is garnered with low gain, then the penalty accrues in excessive lag, resulting in increased door traverse time and additional, though different, irregularities in the door motion due to the lag therein.